A variety of techniques have been developed for collecting and interpreting data concerning the electrical activity of the heart using external medical devices (EMDs) both in the clinical setting, using portable external monitors worn by an ambulatory patient, or outside the clinical setting, using implantable medical devices (IMDs) implanted in an ambulatory patient to collect data relating to electrical heart function during daily activities of the patient. Such techniques include electrocardiography, vectorcardiography and polarcardiography.
The cardiac cycle as displayed in an ECG lead tracing reflects the electrical wave front as measured across an ECG lead, that is between two electrodes spaced apart on the patient's body, as shown in U.S. Pat. No. 4,587,976, for example. The portion of a cardiac cycle representing atrial depolarization is referred to as a “P-wave.” Depolarization of the ventricular muscle fibers is represented by “Q”, “R”, and “S” points of a cardiac cycle. Collectively these “QRS” points are called an “R-wave” or a “QRS complex.” Re-polarization of the depolarized heart cells occurs after the termination of another positive deflection following the QRS complex known as the “T-wave.” The QRS complex is the most studied part of the cardiac cycle and is considered to be the most important for the prediction of health and survivability of a patient. However, the time relation of the P-wave to the QRS complex and the height and polarity of the T-wave and S-segment are also indicators of a healthy or diseased heart. The S-T segment of a healthy heart is usually isoelectric, i.e., neither positive nor negative in deflection from baseline of the ECG lead tracing. This S-T segment is a most important indicator of the health of the ventricular myocardium and is elevated in ischemia and due to infarctions disrupting the depolarization wave front.
The heart rate of the normal heart is governed by the atrial depolarization rate, which is regulated by the body's current requirement for cardiac output reflecting a level of physical exercise or stress. The normal cardiac cycle and heart rate is disrupted in many instances. Conduction defects affecting the A-V node response to a P-wave can cause the ventricles to beat too slowly, that is exhibit bradycardia, and not provide sufficient cardiac output. Other conduction defects and/or disease processes can cause the atria and/or ventricles to spontaneously depolarize at a rapid rate that, that is to exhibit a tachyarrhythmia, that is unrelated to the need for cardiac output, but diminishes or disrupts cardiac output. Such ventricular tachyarrhythmias include ventricular tachycardia (VT), ventricular fibrillation (VF) and ventricular flutter (VFl), and atrial tachyarrthythmias include atrial tachycardia (AT), atrial fibrillation (AF) and atrial flutter (AF).
In AF, the atria depolarize at an elevated rate that is highly irregular, and the atrial depolarizations are typically conducted intermittently to the ventricles, so that the ventricles beat synchronously at times and asynchronously at other times with the atrial depolarizations. In AFl, the atria beat at an elevated rate that is highly regular, and a portion of the atrial depolarizations are typically conducted to the ventricles, whereby the ventricles beat synchronously with every second or third atrial depolarization. Thus, the ventricular heart rate can be in a normal range or elevated but regular during an AFl episode, whereas the ventricular heart rate can be in a normal range or elevated but irregular during an AF episode. Episodes of AF and AFl affect the atrial mechanical function and can have an effect on the ventricular heart rate that negatively affects cardiac output of the ventricles. These episodes are accompanied by faintness, syncope, and tachyarrhythmia palpitation symptoms and occur spontaneously and intermittently.
Moreover, at times, the atria prematurely contract due to depolarizations initiated at ectopic foci other than the SA Node in the atrium, referred to as Premature Atrial Contractions (PACs) or ectopic P-waves. These PACs can be conducted to the ventricles to result in a ventricular contraction or can, due to their amplitude, be mistakenly detected in the ventricles as an R-wave or a ventricular depolarization conducted from the AV node.
Similarly, the ventricles can also develop ectopic foci that intermittently cause a spontaneous depolarization wave front or Premature Ventricular Contractions (PVCs) or ectopic R-waves. Such PACs and PVCs and other arrhythmias can be visually identified by trained medical care providers in the PQRST segments displayed on ECG tracings, if they manifest in the clinical setting.
The ventricular heart rate is determined as a function of the interval between successive ventricular depolarizations each marked by the R-wave of the electrocardiogram (ECG) or electrogram (EGM), that is, the RR interval between successive detected R-waves. Generally, the time interval between successive R-waves is denoted as the RR interval, and the difference between successive RR intervals is denoted as the ΔRR interval. A rapid and regular or irregular ventricular heart rate can be a normal sinus rhythm (NSR) tracking the normal atrial heart rate or can be due to PVCs and/or PACS or conducted AF or AFl or due to VT or VF or VFl originating in the ventricles.
There are many instances where it is desirable to be able to diagnose intermittent spontaneous cardiac arrhythmias, particularly AF and AFl, in ambulatory patients. These episodes of AF and AFl are difficult if not impossible to be induced and observed by the physician in tests conducted in a clinic. There is a recognized need to improve the capability of detecting and distinguishing various types of atrial and ventricular tachyarrhythmias from NSR and one another, so that a drug therapy can be prescribed and so that the efficacy of a prescribed drug therapy can be assessed for efficacy.
For many years, such patients, as well as patients suffering other bradyarrhythmias and tachyarrhythmias, have been equipped with external ECG monitoring systems, e.g., the patient-worn, real time Holter monitors, that continuously sample the ECG from skin electrodes and record it over a certain time period. Then, the ECG data must be analyzed to locate evidence of an arrhythmia episode and its nature and characteristics from which a diagnosis can be made.
As described in commonly assigned U.S. Pat. No. 5,312,446 and in U.S. Pat. No. 4,947,858, both incorporated herein by reference, the externally worn ECG recorders have inherent limitations in the memory capacity for storing sampled ECG and EGM data. Cost, size, power consumption, and the sheer volume of data over time have limited real time external Holter monitors to recording 24-hour segments or recording shorter segments associated with arrhythmias that are felt by the patient who initiates storage.
The use of the externally worn Holter monitor coupled with skin electrodes is also inconvenient and uncomfortable to the patient. The skin electrodes can work loose over time and with movement by the patient, and the loose electrodes generates electrical noise that is recorded with the EGM signal and makes its subsequent analysis difficult. It has long been desired to provide an implantable monitor or recorder that is hardly noticeable by the patient and provides the capability of recording only EGM data correlated with an arrhythmia episode that is automatically detected.
Over the last 40 years, a great many IMDs have been clinically implanted in patients to treat cardiac arrhythmias and other disorders including implantable cardioverter/defibrillators (ICDs) and pacemakers having single or dual chamber pacing capabilities, cardiomyostimulators, ischemia treatment devices, and drug delivery devices. Recently developed implantable pacemakers and ICDs employ sophisticated atrial and/or ventricular tachyarrhythmia detection criteria based on heart rate, rate stability and onset and/or the morphology and other characteristics of the atrial and/or ventricular EGM. Most of these ICDs employ electrical leads bearing bipolar electrode pairs located adjacent to or in an atrial and/or ventricular heart chamber for sensing a near field EGM or having one of the electrodes located on the ICD housing for sensing a far field, unipolar EGM. In either case, the near field or far field EGM signals across the electrode pairs are filtered and amplified in sense amplifiers coupled thereto and then processed for recording the sampled EGM or for deriving atrial and/or ventricular sense event signals from P-waves and/or R-waves of the EGM.
The atrial sense event signals are typically generated by atrial sense amplifiers when the P-wave amplitude exceeds an atrial sense threshold. Similarly, the ventricular sense event signals are typically generated by ventricular sense amplifiers when the R-wave amplitude exceeds a ventricular sense threshold. The ventricular heart rate is typically derived from the measured RR interval between successive ventricular sense event signals.
In current ICDs providing a therapy for treating a cardiac arrhythmia, the sense event signals and certain aspects of the sampled EGM waveform are utilized to automatically detect a cardiac bradyarrhythmia or tachyarrhythmia in one or more heart chamber and to control the delivery of an appropriate therapy in accordance with detection and therapy delivery operating algorithms. In such cardiac ICDs providing pacing or cardioversion/defibrillation therapies, both analog and digital signal processing of the EGM is continuously carried out to sense the P-wave and/or R-wave events and to determine when a cardiac arrhythmia episode occurs. For example, a digital signal processing algorithm is employed to distinguish various atrial and ventricular tachyarrhythmias from one another.
However, the expense and risk from implanting an intracardiac lead and/or a pacemaker with special monitoring functions, such as the utilization of a sense amplifier, is something both patients and physicians would prefer to avoid.
Implantable cardiac monitors have also been developed and clinically implanted that employ the capability of recording cardiac EGM data for subsequent interrogation and uplink telemetry transmission to an external programmer for analysis by a physician. The recorded data is periodically telemetered out to a programmer operated by the medical care provider in an uplink telemetry transmission during a telemetry session initiated by a downlink telemetry transmission and receipt of an interrogation command.
The MEDTRONIC® Reveal™ insertable loop recorder is a form of implantable monitor that is intended to be implanted subcutaneously and has a pair of sense electrodes spaced apart on the device housing that are used to pick up the cardiac far field EGM which in this case is also characterized as a “subcutaneous ECG”. The Reveal™ insertable loop recorder samples and records one or more segment (depending on the programmed operating mode) of such far field EGM or subcutaneous ECG signals when the patient feels the effects of an arrhythmic episode and activates the recording function by applying a patient activator over the site of implantation. For example, the storage of a programmable length segment of the EGM can be initiated when the patient feels faint due to a bradycardia or tachycardia or feels the palpitations that accompany certain tachycardias. The memory capacity is limited, and so the segments of such EGM episode data that are stored in memory can be written over with new EGM episode data when the patient triggers storage and the memory is full. The most recently stored segment or segments of episode data is transmitted via an uplink telemetry transmission to an external programmer when a memory interrogation telemetry session is initiated by the physician or medical care provider using the programmer. Aspects of the Reveal™ insertable loop recorder are disclosed in commonly assigned PCT publication WO98/02209 and in U.S. Pat. No. 6,230,059.
Other examples of external monitoring devices include the Instromedics approach, seen in the Mills, et al patents (U.S. Pat. Nos. 5,333,616; 5,289,824 and 5,111,396) for a wrist worn monitor for ECG's which include features like patient triggering and microprocessor determination of event types (QRS detection). Wrist worn devices are also shown in the Righter patents issued to assignee Ralin, including U.S. Pat. Nos. 5,226,425 and 5,365,935. Jacobsen, et al in U.S. Pat. No. 5,513,645 describes multiple resolution storage for ECG's (ELA Medical is the assignee), and Snell's U.S. Pat. No. 5,518,001 vaguely describes a patient triggered recording device with multiple sensors and patient triggering(assigned to Pacesetter). InControl's approach is seen in the Yomatov patents, U.S. Pat. Nos. 5,411,031 and 5,313,953 which seems to concentrate on beat to beat timing records, suggests the use of an arrhythmia detector, and does mention the possibility of leadless electrodes for monitoring cardiac signals. Examples of an external monitor/recorders can be found in Segalowitz' patents, including U.S. Pat. No. 5,511,553, and Salo's U.S. Pat. No. 5,417,717. Another well known event recorder is the “King of Hearts” (.TM. of Instromedix) which records pre-event and post-event data.
Presently available pacemaker/cardioverter/defibrillator arrhythmia control devices, employ programmable fibrillation interval ranges and tachycardia detection interval ranges, along with measurement of suddenness of onset and rate variability. For future generations of devices, numerous detection and classification systems have been proposed. Numerous patents, including U.S. Pat. No. 5,217,021 issued to Steinhaus et al., U.S. Pat. No. 5,086,772 issued to Larnard et al., U.S. Pat. No. 5,058,599 issued to Andersen and U.S. Pat. No. 5,312,441 issued to Mader et al. propose waveform morphology analysis systems for determining the type and origin of detected arrhythmias. Other patents, including U.S. Pat. No. 5,205,583 issued to Olson, U.S. Pat. No. 5,913,550 issued to Duffin, U.S. Pat. No. 5,193,535 issued to Bardy et al., U.S. Pat. No. 5,161,527 issued to Nappholz et al., U.S. Pat. No. 5,107,850 issued to Olive and U.S. Pat. No. 5,048,521, issued to Pless et al. propose systems for analysis of order and timing of atrial and ventricular events.
In the existing and proposed devices discussed above, one or two basic strategies are generally followed. A first strategy is to identify heart events, event intervals or event rates as they occur as indicative of the likelihood of the occurrence of specific types of arrhythmias, with each arrhythmia having a preset group of criteria that must be met as precedent to detection or classification. As events progress, criteria for identifying the various arrhythmias are all monitored simultaneously, with the first set of criteria to be met resulting in detection and diagnosis of the arrhythmia. A second strategy is to define a set of criteria for events, event intervals and event rates which is generally indicative of a group of arrhythmias, and following those criteria being met, analyzing preceding or subsequent events to determine which specific arrhythmia is present. An arrhythmia detection and classification system generally as disclosed in U.S. Pat. No. 5,342,402, issued to Olson et al., incorporated herein by reference in its entirety, uses both strategies together.
In certain ones of these cardiac monitoring devices, recording of EGM episode data is triggered by the patient. However, in many cases patients are either unaware of “silent” cardiac arrhythmias or are asleep or fail to activate the recording function when they recover from syncope (i.e., have fainted) when bradycardias and tachyarrhythmias occur, and so the accompanying EGM episode data is not recorded. It is therefore desirable to be able to automatically detect an arrhythmia and to initiate recording of the EGM data without having to rely upon the patient as disclosed in the above-incorporated '966 patent. On the other hand, the subcutaneous location environment of the sense electrode pair or pairs on the device housing is relatively noisy due to electromyographic signals generated by adjacent muscle groups that are exercised by the patient. Limb and trunk movements or even breathing can generate noise spikes that are superimposed upon the far field EGM signal and can make it appear to reflect a higher heart rate than the actual heart rate.
While the electromyographic noise level is not as pronounced in relation to the EGM signal level when bipolar sense electrode pairs located in or close by the atrium and ventricle are employed, as is typically the case with bipolar implantable pacemakers and ICDs, so that it is usually possible to filter out such noise in the sense amplifiers of such IMDs, when a cardiac monitoring device that does not include an atrial lead and a sense amplifier is utilized, the only means for reducing the effects of noise is to instruct the patient to assume a quiet body state when he/she initiates recording. As a result, when a cardiac monitoring device that does not include an atrial lead and a sense amplifier is utilized, the ability to differentiate between normal sinus arrhythmia and atrial fibrillation, for example, is even more difficult.
Accordingly, what is needed is a method and apparatus for improving the detection of atrial fibrillation in a cardiac monitoring device that does not utilize an atrial lead and/or atrial sense amplifier.